This invention relates to electron guns for television picture tubes and particularly to electron guns which include extended focus lenses of the resistive type.
As used herein, the term resistive lens means an electrostatic focus lens in which the potential profile of the lens is established by a resistive voltage divider along the length of the lens. One type of such lens disclosed in U.S. Pat. No. 3,932,786, issued to F. J. Campbell on Jan. 13, 1976, comprises a series of apertured metal plates which are connected to spaced taps along the voltage divider. The apertured plates are supported in fixed relationship by embedding their edges along a glass support rod which also serves as a substrate for the voltage divider resistor deposited thereon.
A modification of the Campbell-type of resistive lens is disclosed in U.S. Pat. No. 4,091,144, issued to J. Dresner et al on May 23, 1978, and in a copending application, Ser. No. 51,400, filed June 25, 1979 by B. Abeles. In this modification, the apertured plates are alternately stacked with a plurality of insulator blocks, e.g. ceramic, which are coated on at least one face with a resistive material. The plates and blocks are so arranged that a high resistance continuity is established along the stack of blocks and plates from one end to the other. When a potential difference is applied across the stack, current flow is created which results in each electrode plate of the stack having a different voltage applied to it.
U.S. Pat. No. 4,124,810, issued Nov. 7, 1978 to D. P. Bortfeld et al, teaches the desirability of the potential profile of a focus lens being exponential-like along the beam path. This can be achieved in the Abeles-type resistive lens simply by grading the resistance values of successive blocks along the lens stack. However, such a procedure is costly and complex in that each resistive block must be pretested and selected as to its specific resistance, and then these blocks must be carefully handled so that the blocks are correctly assembled in their proper order in the resistive stack.
For purposes of simplicity herein, no distinction is made herein between axial potential profile of a lens, i.e., the potential profile along the electron beam axis through the lens, and surface potential profile of a lens, i.e., the potential profile along the surfaces of the electrode elements of the lens in the axial direction. In practice they differ slightly, the axial profile usually being a smoothed art replica of the surface profile.
We have discovered that the optimum exponential-like voltage profile of a lens can be closely approximated by two linear slopes (i.e., linear voltage gradients) without greatly increasing lens aberrations. Computer analysis has shown these two linear slopes to have an optimum ratio of about 1:2 to 1:3. Furthermore, we have discovered that the values of these two linear slopes can be established, preferably with a 1:2 ratio without significant degradation of performance, e.g., in a tripotential type of lens system, such that a resistive lens according to the Abeles teaching can be constructed using only one value of resistive block, thereby greatly simplifying and cost-reducing the construction of such lenses.
As used here, the term "tripotential" describes a lens system comprising at least three electrodes, the first of which along the beam path is operated at an intermediate potential, the second at a minimum potential, and the third at the ultor or screen potential of the electron tube incorporating the lens. Electron guns having axial potential profiles of this general class are disclosed in U.S. Pat. No. 3,995,194, issued to A. P. Blacker, Jr. et al on Nov. 30, 1976.
The terms "resistive lens stack" and "resistive lens structure" are used interchangeably herein, and means either:
(a) a portion of a total lens stack comprising a series of electrode plates and one aligned row of resistive blocks, or
(b) the entire lens stack comprising a series of electrode plates and all resistive blocks thereof including those situations where a portion of the stack has two or more resistive blocks in each stage thereof.